From: aansar@sscomp.com.sg
Date: Fri Apr 25 2003 - 00:31:44 GMT-3
Brian McGahan(ipexpert)posted this some time back.. hope it will help to
understand what prefix list can do..
Prefix-lists are used to match on prefix and prefix-length pairs. Normal
prefix-list syntax is as follows:
Ip prefix-list LIST permit w.x.y.z/len
Where w.x.y.z is your exact prefix
And where len is your exact prefix-length
-----------------------------
"Ip prefix-list LIST permit 1.2.3.0/24" would be an exact match for the
prefix 1.2.3.0 with a subnet mask of 255.255.255.0.
This does not match 1.2.0.0/24, nor does it match 1.2.3.4/32, nor anything
in between.
-----------------------------------
When you add the keywords "GE" and "LE" to the prefix-list, the "len" value
changes its meaning. When using GE and LE, the len value specifies how
many bits of the prefix you are checking, starting with the most
significant bit.
-------------------------------
Ip prefix-list LIST permit 1.2.3.0/24 le 32
This means:
Check the first 24 bits of the prefix 1.2.3.0
The subnet mask must be less than or equal to 32
This equates to the access-list syntax:
Access-list 1 permit host 1.2.3.0
---------------------------------
Ip prefix-list LIST permit 0.0.0.0/0 le 32
This means:
Check the first 0 bits of the prefix 0.0.0.0
The subnet mask must be less than or equal to 32
This equates to anything
---------------------------------------
Ip prefix-list LIST permit 0.0.0.0/0
This means:
The exact prefix 0.0.0.0, with the exact prefix-length 0.
This is matching a default route.
---------------------------------------
--------------
Ip prefix-list LIST permit 10.0.0.0/8 ge 21 le 29
This means:
Check the first 8 bits of the prefix 10.0.0.0
The subnet mask must be greater than or equal to 21, and less than or
equal to 29.
------------------------------
Ip prefix-list CLASS_A permit 0.0.0.0/1 ge 8 le 8
This matches all class A addresses with classful masks.
means:
Check the first bit of the prefix, it must be a 0.
The subnet mask must be greater than or equal to 8, and less than or
equal to 8. (It is exactly 8)
-------------------------------------------------------
When using the GE and LE values, you must satisfy the condition:
Len < GE <= LE
Therefore:
Ip prefix-list LIST permit 1.2.3.0/24 ge 8
Is not a valid list.
-------------------------------------------------------------------
What you can not do with the prefix-list is match on arbitrary bits like
you can in an access-list. Prefix-lists cannot be used to check if a
number is even or odd, nor check if a number is divisible by 15, etc...
Bit checking in a prefix-list is sequential, starting with the most
significant (leftmost) bit.
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